Refactor assistant configuration management and update documentation

- Removed legacy agent profile settings from the .env.example and README, streamlining the configuration process.
- Introduced a new local YAML configuration adapter for assistant settings, allowing for easier management of assistant profiles.
- Updated backend integration documentation to clarify the behavior of assistant config sourcing based on backend URL settings.
- Adjusted various service implementations to directly utilize API keys from the new configuration structure.
- Enhanced test coverage for the new local YAML adapter and its integration with backend services.

engine/services/openai_compatible_asr.py

@@ -75,7 +75,7 @@ class OpenAICompatibleASRService(BaseASRService):
         if not AIOHTTP_AVAILABLE:
             raise RuntimeError("aiohttp is required for OpenAICompatibleASRService")
         
-        self.api_key = api_key or os.getenv("ASR_API_KEY") or os.getenv("SILICONFLOW_API_KEY")
+        self.api_key = api_key
         raw_api_url = api_url or os.getenv("ASR_API_URL") or self.API_URL
         self.api_url = self._resolve_transcriptions_endpoint(raw_api_url)
         self.model = self.MODELS.get(model.lower(), model)